When solid-state crystals are subjected to ionizing radiation, several absorption bands make their appearance at increasingly higher levels of radiation damage.sup.(1). In the case of the alkali halides, the F-band is the first of the radiation damage centers to produce a detectable absorption band. Although the F-center provides the greatest concentration of damage centers for a given radiation exposure, its physical properties preclude its use in quantifying ionizing radiation exposure using luminescence techniques. The luminescence of the F-center is thermally quenched below room temperature.
With increasing radiation exposure, a second damage center builds up in the absorption spectrum. The second damage center is known as the M-center and is generally thought to consist of two adjacent F-centers.sup.(2,3). Unlike the F-center, however, excitation into the longer wavelength M-center absorption band at room temperature produces luminescence.sup.(4). M-center luminescence involves a Stokes' shift, allowing the luminescence to be observed at a significantly different wavelength from the exciting wavelength.
Highly purified lithium fluoride (LiF) crystals have long been used as optical windows. Optical grade LiF is known for its excellent transmission mission from the deep ultraviolet through the infra-red. Radiation dosimetry applications have so far been restricted to high-level Mega-Rad gamma dosimetry using radiation-induced absorption peaks measured with a spectrophotometer. However, absorption measurements are a very insensitive way to measure these radiation damage centers.